This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We study aquaporin 0 (AQP0), a transmembrane water channel from the eye lens, causing severe lesions in the lens, cataracts and blindness when mutated. AQP0 regulates water permeability within the eye lens by responding to changes in [Ca2+] through an interaction with calmodulin. Calmodulin (CaM) is a ubiquitous cytosolic protein commonly coupled with Ca2+ regulation. We suggest the Ca2+/CaM interaction with AQP0 lead to major conformational changes within the water channels that enfluence water permeability rates. The aims of this proposal are to use X-ray crystallography to determine the high-resolution structure of the AQP0-CaM complex. We can fatihfully grow large crystals containing both AQP0 and CaM, however obtaining high-resolution structural information has been stymied by poor diffraction. Success in achieving our aim will have wide implications in the fields of water channel regulation, as well as broader implications to the Ca2+/CaM signaling system and to furthering membrane structural biology in general.